Wildlife Gardening – Ecology of a Wildlife Garden
N.B. If your new to David’s monthly column then please be sure to read the ‘setting-the-scene’ opening piece first.
When our monthly pay arrives in our bank accounts the kindly government has already diverted a fair proportion to their own bank. They call it tax and, try as we might to demand that it is returned to us, we seldom succeed. To make matters worse when we travel, to spend what has been left, they take even more! Petrol tax, car tax and even VAT on virtually everything in the shops. At each and every stage the hard earned money is diverted into governmental coffers. Even if the money is cached away in a bank account the interest attracts tax. We cannot win.
It is just the same in the garden. The sun’s generous supply of energy reaches the Earth as sunlight energy, which can be employed for many useful purposes, but much of it is diverted elsewhere. This free sunlight energy is absorbed by atmospheric gases, reflected off clouds or dust, and so is reduced in quantity (by 30% or more) even before it reaches ground level. Now some of the remaining energy is lost to the environment for it evaporates surface water, heats the soil or is just unable to be captured by plants.
Green plants are simply amazing for they, together with some algae and green bacteria, are able to convert sunlight energy into chemical energy.
Carbon dioxide and water will not burn to release energy. But green plants (often biologically called producers) can take these same energy-poor raw materials and, using the energy in sunlight, combine them together into energy-rich sugars and fats. Photosynthesis.
Photosynthesis is grossly inefficient and generally works at only a 1% efficiency level, and it is very frequently well below even this lowly level. All the green light that falls onto leaves is simply reflected or passes through uncaptured, but that small percentage that is fixed into a chemical form drives the huge biodiversity that we call ‘life on earth’.
Some environments are more efficient than others at energy capture via photosynthesis, and it is these that are usually the richest biologically. At the very top is a moist tropical evergreen forest, then tropical estuaries and swamps, and at the very bottom desert scrub and extreme deserts.
To put figures to this for comparison;
Moist tropical forest – 36160 (Kilojoules per metre squared per year)
Industrialised agriculture – 12290
UK woodland – 11340
Continental shelf oceans – 6620
Open oceans – 2420
Desert scrub – 1320
How would you rate the following UK environments in terms of energy capture?
Inter-tidal mud flats
Heath or moorland
Use the UK woodland figure as a guide.
This captured energy can now be used to grow the plant (10%) but much will be lost (90%) in shed leaves or bark, or in maintaining its life processes. So not all these energy-rich chemicals are available as ‘food’ for other organisms (usually, biologically, called consumers). Energy is lost in the chain of life just like our earnings to the taxman or woman.
Eating tough, and relatively difficult to digest, plants is quite a specialised task, so not all animals can achieve it. Those that can are the herbivores, but they too are inefficient in capturing the energy in plant food.
A caterpillar gains only 23% of the energy contained in the food it eats, the rest is lost. And that’s good! An elephant achieves only a 1% transfer … but you’d have guessed that if you’ve ever seen elephant poo!
You’ve got the message now … at each stage in this system (food chain) energy is lost, until it is all gone. The poor old carnivores have to expend so much energy in finding, catching and consuming their prey animals that there is virtually little left for growth.
As each step in the food chain produces waste, or when an animal dies, some energy is diverted to the decomposers such as fungi or termites etc. In fact, after plants, these decomposers are the most common organisms in terms of their weight.
How about UK meat production?
Place the following in order of efficiency in conversion of food into meat (in a controlled farming environment).
Ecologists often count the number of each type of organism (producer, herbivore, carnivore or decomposer) but assessing their weight (biomass) is much more accurate as beasties vary in weight.
Sensible wildlife enthusiasts are plant hunters, for producers are relatively common, with a high biomass. Decomposers come next, followed by herbivores and finally carnivores. Only silly folks specialise in big carnivores for they are relatively uncommon in comparison to other organism groups. (But they are so impressive it ensures the effort is worthwhile!)
This pattern will be found in a typical garden: plenty of plants; the often-unseen fungi massing in the soil and decaying vegetation in a big biomass; herbivorous snails, crickets and voles in modest numbers and biomass; predators and top predators with a low biomass and relatively uncommon.
Most organisms are specialists; they cannot live everywhere nor can feed off everything. Their special ‘location’ or ‘role’ in an environment is their niche. For example a large white butterfly larva’s niche will be: herbivore of cabbage leaves. A green woodpecker: carnivore of ants. Ivy broomrape: parasite of ivy plants. Each organism is often quite highly specialised, but there are exceptions and brown rats have a wider niche than perhaps many other mammals ….. but even they would have trouble tackling a live cow for food!
Next month we’ll explore the concept of niche in the garden more fully.
David is an ex-lecturer in Environmental / Biological Sciences.
He has written for most of the UK gardening magazines, including the RHS.
Forest Edge’s garden has been widely covered in magazines and has been featured in a BBC’s Gardeners’ World special.
He also maintains a blog at https://nwhwildlife.org/